Your voltmeter is probably inaccurate. The voltage reference for the TPS is supposed to be 5V, so no voltage higher than that should come out out of it. Either that or the voltage reference (on one of the other two wires) is wrong, but that's highly unlikely.
There was much discussion on this mod on Corral.net a while ago, and someone said that it doesn't do very much because the PCM considers the lowest voltage it gets from the TPS to be the fully-closed throttle position. Nobody has ever been able to explain why .98V happens to be the "correct" reading, either...
